Radio frequency identification (RFID) systems and other forms of electronic article surveillance are increasingly used to track items whose locations or dispositions are of some economic, safety, or other interest. In these applications, typically, transponders or tags are attached to or placed inside the items to be tracked, and these transponders or tags are in at least intermittent communication with transceivers or readers which report the tag (and, by inference, item) location to people or software applications via a network to which the readers are directly or indirectly attached. Examples of RFID applications include tracking of retail items being offered for public sale within a store, inventory management of those items within the store backroom, on store shelving fixtures, displays, counters, cases, cabinets, closets, or other fixtures, and tracking of items to and through the point of sale and store exits. Item tracking applications also exist which involve warehouses, distribution centers, trucks, vans, shipping containers, and other points of storage or conveyance of items as they move through the retail supply chain. Another area of application of RFID technology involves asset tracking in which valuable items (not necessarily for sale to the public) are tracked in an environment to prevent theft, loss, or misplacement, or to maintain the integrity of the chain of custody of the asset. These applications of RFID technology are given by way of example only, and it should be understood that many other applications of the technology exist.
In the case of passive RFID systems, the RFID tag is powered by the electromagnetic carrier wave. Once powered, the passive tag interprets the radio frequency (RF) signals and provides an appropriate response, usually by creating a timed, intermittent disturbance in the electromagnetic carrier wave. These disturbances, which encode the tag response, are sensed by the reader through the reader's antenna. In the case of active RFID systems the tag contains its own power source, such as a battery, which it can use to either initiate RF communications with the reader by creating its own carrier wave and encoded RF signals, or else the tag power can be used to enhance the tag performance by increasing the tag's data processing rate or by increasing the power in the tag's response, and hence the maximum distance of communication between the tag and reader.
RFID systems typically use reader antennas to emit electromagnetic carrier waves encoded with digital signals to RFID tags. As such, the reader antenna is a critical component facilitating the communication between tag and reader, and influencing the quality of that communication. A reader antenna can be thought of as a transducer which converts signal-laden alternating electrical current from the reader into signal-laden oscillating electromagnetic fields or waves appropriate for a second antenna located in the tag, or alternatively, converts signal-laden oscillating electromagnetic fields or waves (sent from or modified by the tag) into signal-laden alternating electric current for demodulation by and communication with the reader. Types of antennas used in RFID systems include patch antennas, slot antennas, dipole antennas, loop antennas, and many other types and variations of these types.
The detection range of passive RFID systems is typically limited by signal strength over short ranges, for example, frequently less than a few feet for passive UHF RFID systems. Due to this read range limitation in passive UHF RFID systems, many applications make use of portable reader units or mobile carts with readers and antenna wands tethered to the readers with cables. These portable or mobile reader systems may be manually moved around a group of tagged items in order to detect all the tags, particularly where the tagged items are stored in a space significantly larger than the detection range of a stationary or fixed reader equipped with one fixed antenna. However, portable UHF reader and antenna units suffer from several disadvantages. The first involves the cost of human labor associated with the scanning activity. Fixed infrastructure, once paid for, is much cheaper to operate than are manual systems which have ongoing labor costs associated with them. In addition, portable units often lead to ambiguity regarding the precise location of the tags read. For instance, the reader location may be noted by the user, but the location of the tag during a read event may not be known sufficiently well for a given application. That is, the use of portable RFID readers often leads to a spatial resolution certainty of only a few feet, and many applications require knowledge of the location of the tagged items within a spatial resolution of a few inches. Portable RFID readers and mobile reader carts can also be more easily lost or stolen than is the case for fixed reader and antenna systems.
As an alternative to portable UHF RFID readers, a large fixed reader antenna driven with sufficient power to detect a larger number of tagged items may be used. However, such an antenna may be unwieldy, aesthetically displeasing, and the radiated power may surpass allowable legal or regulatory limits. Furthermore, these reader antennas are often located in stores or other locations were space is at a premium and it is expensive and inconvenient to use such large reader antennas. In addition, it should be noted that when a single large antenna is used to survey a large area (e.g., a set of retail shelves, or an entire cabinet, or entire counter, or the like), it is not possible to resolve the location of a tagged item to a particular spot on or small sub-section of the shelf fixture. In some applications it may be desirable to know the location of the tagged item with a spatial resolution of a few inches (e.g., if there are many small items on the retail shelf and it is desired to minimize manual searching and sorting time). In this situation the use of a single large reader antenna is not desirable because it is not generally possible to locate the item with the desired spatial resolution.
Alternatively, a fully automated or mechanized antenna system can be used. U.S. Pat. No. 7,132,945 describes a shelf system which employs a mechanized scanning antenna. This approach makes it possible to survey a relatively large area and also eliminates the need for human labor. However, the introduction of moving parts into a commercial shelf system may prove impractical because of higher system cost, greater installation complexity, and higher maintenance costs, and inconvenience of system downtime, as is often observed with machines which incorporate moving parts. Beam-forming smart antennas can scan the space with a narrow beam and without moving parts. However, as active devices they are usually big and expensive if compared with passive antennas.
To overcome the disadvantages of the approaches described above, fixed arrays of small antennas are utilized in some UHF RFID applications. In this approach numerous reader antennas spanning over a large area are connected to a single reader or group of readers via some sort of switching network, as described for example in U.S. Pat. No. 7,084,769. Smart shelving and other similar applications involving the tracking or inventory auditing of small tagged items in or on RFID-enabled shelves, cabinets, cases, racks, or other fixtures can make use of fixed arrays of small antennas. In tracking tagged stationary items in smart shelving and similar applications, fixed arrays of small antennas offer several advantages over portable readers, systems with a single large fixed antenna, and moving-antenna systems. First, the antennas themselves are small, and thus require relatively little power to survey the space surrounding each antenna. Thus, in systems which query these antennas one at a time, the system itself requires relatively little power (usually much less than 1 watt). By querying each of the small antennas in a large array, the system can thus survey a large area with relatively little power. Also, because the UHF antennas used in the antenna array are generally small and (due to their limited power and range of less than 1-12 inches) survey a small space with a specific known spatial location, it must also be true that the tagged items read by a specified antenna in the array are also located to the same spatial resolution of 1-12 inches. Thus systems using fixed arrays of small antennas can determine the location of tagged items with more precision than portable RFID readers and systems using a small number of relatively large antennas. Also, because each antenna in the array is relatively small, it is much easier to hide the antennas inside of the shelving or other storage fixture, thus improving aesthetics and minimizing damage from external disruptive events (e.g., children's curiosity-driven handling, or malicious activity by people in general). Also, an array of fixed antennas involves no moving parts and thus suffers from none of the disadvantages associated with moving parts, as described above. Also, small antennas like those used in such antenna arrays may be cheaper to replace when a single antenna element fails (relative to the cost of replacing a single large antenna). Also, fixed arrays of antennas do not require special manual labor to execute the scanning of tagged items and, therefore, do not have associated with them the high cost of manual labor associated with portable reader and antenna systems, or with mobile cart approaches.
Almost without exception, implementations of RFID technology involve the direct connection of antennas to an RFID reader, and thus limit the antenna-to-reader ratio to a relatively low number (almost never greater than four). Occasionally, implementations involve the use of multiplexing switches between the reader and the antennas, allowing for a larger number of antennas for each reader. FIG. 1 is a schematic illustrating a typical prior art approach. Individual RFID antennas 100 are connected to a central common RF communications cable 105 using simple switches or relays 110. Over the common cable, the antennas are driven from an RFID reader 120 which generates outgoing and interprets incoming RF signals, referred to herein as “RFID traffic signals” or just “traffic signals”. Here RFID traffic signals deals specifically with the signals used to communicate between RFID readers and tags, but in some cases specifically noted in this document “RFID traffic” could also include device control and command signals. Unless otherwise stated, in the descriptions below “RFID traffic” refers to signals to and from antennas for communication with RFID tags. In FIG. 1 the reader is controlled by commands received from a computer 130. To initiate communication with tags (or transponders) 140 within the read range of a particular antenna, the computer 130 selects an antenna and sends the identity of the selected antenna to the switch controller 150, which in turn activates the selected antenna using a control line 115 coupled between the switch controller 150 and the antenna's associated relay 110. The other antennas are deactivated over their respective control lines. The computer 130 then instructs the reader 120 to collect the required information, and the results from the reader 120 are returned to the computer 130 and associated with the active antenna.
Even though the approach shown in FIG. 1 allows the use of many antennas with a single or small number of readers, the technology used to control the multiplexers is crude, requiring manual configuration of the network, and not allowing failover from one reader to another when a reader on the network is disabled. That is, these crude network implementation based on simple multiplexers involve the direct assignment of each antenna to a specific, single reader, and rely upon the health of that single reader for its operation.
The practical implementation of large arrays of small antennas using only a small number of readers depends upon a robust, simple, and economical signal routing approach. The current implementations described herein deal with an RF network control module and method for creation of a much more robust network in which each antenna can be accessed by any one of a collection of two or more readers, depending upon need. In prior art RFID antenna networks, each antenna is assigned to a particular reader and can be accessed by no other reader in the network. If a reader fails or goes off line for any reason, all of the antennas assigned to that reader are essentially dead to the network. Using the prior art, the only way to make it possible to access a particular antenna from more than one reader is to use complex combinations of multiplexers, separate control lines, and external switches. The current embodiments replace all of those components with a single device which allows multiple RFID readers access to the same set of antennas, thus providing the reader failover capability (i.e., a reader failure is detected by the host system managing the network, and is replaced by an active reader such that all antennas in the network remain accessible). Furthermore, a great advantage of the current embodiments over complex combinations of multiplexers, control lines, and external switches is that the current embodiments of the device can be controlled over the same lines that are used to carry the RFID traffic for communication with RFID tags. This greatly minimizes the cabling or wiring requirements for the network, providing lower cost, shorter installation times, easier maintenance, better aesthetics, smaller space requirements, and a number of other advantages. Because the current embodiments makes it practical to introduce redundant pathways in the network, allowing multiple readers to access a given antenna, it allows for network loading balancing. That is, the RFID network host system managing the readers can track the use of readers (load on readers) and use the switching capabilities of the device described in the current invention to spread the load evenly over the readers assigned to a given area of activity in the network.